Rolling is an operation for the shaping of metals by plastic deformation. It is intended to reduce the thickness of a strip by passing it through two or more pairs of axially symmetrical tools rotating about their axes (typically rolls). Their rotation draws the product through by friction in the gap provided by the entry, working and exit zones. Longitudinal tensile forces (on exit) and opposing tensile forces (on entry) applied simultaneously can be used to reduce the normal force imposed by the rolls (gripping force).
After passing through hot rolling, cold rolling can be used to provide a product with a precise geometry, controlled mechanical and metallurgical properties and a well-controlled surface condition. The roughness obtained results from transfer of the roughness of the tool onto the strip. It is highly dependent on lubrication. Some strips have to be delivered bright, therefore smooth (with a roughness of close to 0.2 μm). In order to manufacture a smooth strip ground or even polished rolls are used, together with a low viscosity lubricant. The surface conditions of rolled strips may show irregularities, for example gouges resulting from local breakdown of the lubricant film causing metal particles to be torn off and adhere to the rolls.
From a chemical point of view, corrosion phenomenon may appear. The presence of reactive films or contamination of the strips by lubricants from previous operations are also difficulties which have to be overcome. Residues may mark the surface when rolled strip is annealed.
FIG. 1 shows a metal strip at the entry to a rolling mill. In the entry zone the strip of initial thickness e1 is drawn in at a speed v1 by two rolls. It is plastically deformed in the working area and leaves the roll gap with a thickness e2. Because the quantity of material is conserved, the strip is accelerated in the gap as it is reduced and elongates. Thus the exit speed v2 of the strip is faster than the entry speed v1. The level of reduction is defined by r=(e1−e2)/e1. In an normal rolling pass there is a “neutral point” in the gap where the peripheral speed of the rolls is the same as the local speed of the strip. Its position will depend on the longitudinal tensions applied to the strip, the friction conditions, the reduction and the rolling speed.
Thus except for small zone around the neutral point there is relative slip between the strip and roll in the gap, and therefore friction and shear stresses at the interface. Upstream from the neutral point the rolls tend to draw the strip into the gap, friction is the driver.
The friction must be sufficient to allow the strip to be effectively fed into the gap, but not excessive in order to avoid any sticking or problems with surface condition. It is essential that a lubricant should be present to control friction and as a consequence irregularity in the thickness and surface condition of the strip. It is therefore important that lubricant behaviour should be well controlled at the scale of the contact between the rolls and the strip for the better control of friction during cold rolling.
Upstream of the neutral point the rolls tend to draw the strip into the gap, friction is the driver. Mixed elastohydrodynamic (EHD) friction conditions are found in this upstream zone: in the more upstream zone the lubricant film is continuous, wear is low, there is no contact between the rough points on the two opposing surfaces, but pressures generated in the film are sufficiently high to cause significant elastic deformation of the surfaces. The type of friction generated in this zone can be reproduced in an EHD ball-on-disc tribometer.
Downstream of the neutral point slip opposes advancement of the strip: friction provides resistance. Limiting friction conditions prevail in this zone. Unlike in EHD lubrication, limiting lubrication is a regime in which friction and wear of the two surfaces in relative movement are jointly determined by the properties of the solid surfaces and those of the lubricant. Thus the thickness and nature of the oxide layers, the creation of fresh surface and its reactivity to components of the lubricants, in particular additives, have an enormous effect on friction. This type of friction can be reproduced in a Cameron Plint cylinder-on-flat tribometer.
The behaviour of rolling fluids upstream and downstream of the neutral point (in the gap) is mainly governed by the bases and fatty substances in the upstream part, where EHD and mixed friction conditions prevail. The addition of additives, in particular extreme pressure additives, together with the bases and fatty substances, makes a significant contribution to the performance of these fluids in the downstream part, where limiting lubrication conditions exist. Work on rolling oils essentially relates to the nature of the fatty substances or synthetic esters added to the compositions. Fluids in the prior art are not optimised in respect of bases and additives.
Thus application EP 1 123965 describes a fluid for the cold rolling of steel comprising a naphthenic or paraffinic base oil, which may or may not be hydrocracked, which may have been freed of aromatics using solvent or by hydrotreating, the viscosity of which can be adjusted by kerosene cuts and from 1 to 80% di(2-ethylhexyl)adipate as the fatty body. The composition may also contain phosphorus-, sulphur- or phosphorus/sulphur-containing anti-wear and extreme pressure additives. Patent EP 0242 925 describes rolling fluids containing esters of amino alcohols and fatty acids comprising at least 6 carbon atoms. No extreme pressure additives are disclosed in this document, and no information is provided about the nature of the mineral bases employed in the said fluids.
No specific choice of bases or additives is made in the rolling fluids of the prior art to ensure optimum friction properties under EHD and/or limiting conditions. It follows that reducing the thickness of strip by a particular amount requires a number of rolling passes with these fluids. Use of these fluids according to the prior art may also result in sticking, which means that rolling can no longer take place, or microsticking which has an adverse effect on surface condition.
There is therefore a need for oils which can improve the productivity of rolling mills, for example by reducing the number of passes required in order to obtain a given amount of reduction without sticking or microsticking which might have an adverse effect on the surface condition and in particular the brightness of the strips. Rolling fluids according to the invention have friction coefficients under elastohydrodynamic conditions that are significantly lower on steel surfaces than those of the lubricants currently used in cold rolling. Thus, the neutral point is moved downstream, which results in higher levels of reduction per pass and makes it possible to reduce the total number of passes in order to achieve a given amount of reduction. This can improve the performance of the rolling mill.
In particular, when they contain phosphorus- and/or phosphorus/sulphur- and/or sulphur-containing extreme pressure additives fluids according to the invention also have friction properties under limiting conditions on steel surfaces which are superior to those of the fluids in the prior art. In fact, friction coefficients obtained on a Cameron Plint tribometer using such fluids according to the invention show that on ferritic and austenitic steel surfaces significantly lower friction coefficients are obtained up to higher temperatures and under higher loads (thus under more severe friction conditions) than with commercial lubricants for the cold rolling of steel.
Under heavy load commercial lubricants give rise to immediate sticking, whereas using lubricants according to the invention friction is observed under heavy load up to temperatures of the order of 100° C. Thus, the risks of sticking are considerably reduced and heavier loads can be applied to the rolls and large reductions in thickness can be obtained with a smaller number of passes. This further helps to improve the efficiency of rolling mills, and also improves the surface condition of the rolled strip. This is more advantageous in that these very good properties can be obtained with rolling fluids according to the invention which have a low sulphur content, or are even sulphur-free.
Additives containing sulphur have a very effective action on friction properties, particularly under limiting conditions, but have a tendency to form iron sulphide on the fresh surfaces produced by rolling, which results in the strips becoming marked. These marks can be removed by annealing at 1200° C. in the case of austenitic steel but H2S is formed, causing corrosion of furnace refractories. In the case of ferritic steel annealing at 900° C. is insufficient to remove marks.
Rolling fluids according to the invention containing particular phosphorus-containing additives in possible combination with phosphorus/sulphur- and/or sulphur-containing additives have very good friction properties with low levels of sulphur, and even in the absence of sulphur. Thus, according to one embodiment rolling fluids according to the invention can increase the efficiency of rolling mills and improve the surface condition and therefore the brightness of rolled strip, avoiding the risk of marking the strip.
This invention relates to a cold rolling fluid comprising:
(a) a hydrocarbon base comprising at least 50% by weight of isoparaffins,
(b) one or more fatty substances, preferably selected from the esters of fatty acids, or polymer esters obtained by the esterification of alpha olefin and dicarboxylic acid copolymers by alcohols.
Preferably, in the rolling fluid according to the invention the hydrocarbon base (a) comprises at least 60% by weight of isoparaffins. Preferably, in the fluid the hydrocarbon base (a) comprises petroleum cuts having an initial and final distillation point of between 200 and 400 measured according to ASTM D86, comprising hydrocarbon molecules having between 13 and 25 carbon atoms. Preferably, hydrocarbon bases (a) have an aromatics content of not more than 100 ppm, and a sulphur content of not more than 1 ppm measured according to ASTM D2622.
According to one embodiment the fluid according to the invention also comprises:
(c) one or more phosphorus- and/or phosphorus/sulphur- and/or sulphur-containing anti-wear and/or extreme pressure additives.
Preferably, the rolling fluid according to the invention has a sulphur content of less than 1100 ppm according to standard ASTM D 2622, preferably less than 1000 ppm, preferably less than 500 ppm.
According to a preferred embodiment, the rolling fluid according to the invention comprises one or more organophosphorus compounds derived from phosphoric and/or phosphorous acids as compound (c), the said fluid having a phosphorus content measured according to standard NFT 60-106 of at least 500 ppm, with the condition that when the said fluid only contains phosphoric acid derivatives by way of compound(s) (c) it contains at least one sulphur- or phosphorus/sulphur-containing compound and its sulphur content measured according to ASTM standard D2622 is at least equal to 300 ppm. According to a preferred variant the rolling fluid according to the invention comprises at least one phosphorous acid derivative by way of compound (c) and a sulphur content of less than 300 ppm when measured according to ASTM D2622.
Preferably, the VI (viscosity index) of the rolling fluid according to the invention measured according to ASTM standard D2270 is greater than 110, preferably greater than 120, preferably greater than 130. Its kinematic viscosity at 100° C. measured according to ASTM D445 preferably between 2 and 3, preferably between 2.5 and 2.65 cSt. Its kinematic viscosity at 40° C. measured according to ASTM D445 is preferably between 7.5 and 9, preferably between 7.6 and 8.8 cSt.
According to one embodiment the rolling fluid according to the invention comprises:                50 to 90% by weight of a hydrocarbon base (a)        5 to 20% by weight of one or more fatty substances (b)        0.5 to 7% by weight of one or more organophosphorus compounds derived from phosphoric and/or phosphorous acids (c).According to one embodiment the rolling fluid according to the invention may also comprise 1 to 20% by weight of one or more surfactants, preferably selected from non-ionic or anionic surfactants.        
The present invention also relates to an aqueous emulsion comprising the latter rolling fluid. The present invention also relates to use of a rolling fluid or an aqueous emulsion according to the invention for the cold rolling of steel, preferably austenitic or ferritic steel.